The present invention relates to optical communication devices and more particularly concerns a gain-locked, dual or multi-stage optical amplifier.
In WDM communication systems containing optical amplifiers, adding or dropping one or several channels can cause high fluctuations in the remaining channels that may be detrimental to the system operation. In the case of channel dropping, the surviving channels will experience a power increase up to several decibels that may give rise to higher bit-error-rate (BER) due to receiver overload or to nonlinear phenomena such as stimulated Brillouin scattering if the power threshold is exceeded. When channels are added, the main problem is a degraded BER due to a decreased channel power at the receiver. It is therefore necessary to provide means for maintaining a constant WDM gain spectrum under various conditions.
Several techniques have been proposed for this purpose. Known in the art are the publications of K. Motoshima et al, xe2x80x9cDynamic compensation of transient gain saturation in erbium-doped fiber amplifiers by pump feedback controlxe2x80x9d, Tech. Dig. OFC""93, pp. 40-42 (1993), and A. K. Srivastava et al, xe2x80x9cFast gain control in an erbium-doped fiber amplifierxe2x80x9d, Tech. Dig. OAA""96 pp. 24-27 (1996), where a probe signal at the output of the amplifier is sampled, and its power is kept constant by adjusting the pump laser power when input channels are added or dropped. The main drawback of these methods is that it involves having one channel dedicated to gain locking.
All-optical gain-clamped amplifiers were also demonstrated (see for example H. Dai et al., xe2x80x9cAll-optical gain control of In-line erbium-doped fiber amplifiers for hybrid analog/digital WDM systemsxe2x80x9d, IEEE Photon. Tech. Lett., vol 9, pp. 737-739 (1997) and J. F. Massicott et al. xe2x80x9c1480 nm pumped erbium-doped fibre amplifier with all optical automatic gain controlxe2x80x9d, Electron. Lett. vol 30, pp. 962-964 (1994)). In this method, the population inversion, hence the gain, is clamped by lasing action at a wavelength outside of the amplification band. Both linear and loop configurations were proposed. The main drawbacks of this type of method is the presence, if not filtered, of an undesirable laser signal at the output of the amplifier and the required additional optical components that may increase the noise figure of the amplifier. Furthermore this technique is very sensitive to the laser wavelength stability and laser cavity loss.
There is also known in the art a technique to achieve gain locking using the pump power loss monitoring as disclosed in M. Karasek xe2x80x9cAnalysis of dynamic pump-loss controlled gain-locking system for erbium-doped fiber amplifiersxe2x80x9d, IEEE Photon. Tech. Lett. vol 10, pp. 1171-1173 (1998). In this technique, the gain is locked by maintaining constant the ratio of the launched pump power to the unabsorbed pump power, as illustrated in the enclosed FIG. 1 (prior art). Advantageously, such a method requires only a few additional optical components and a simple electronic circuitry for the feedback loop. Karasek et al. used this technique in a single stage amplifier configuration and good results were obtained. In this technique, the gain is locked to the desired value by keeping constant the ratio of the launched pump power to the unabsorbed pump power, the launched pump power being therefore given by the equation:
Plaunched=kxc2x7Punabsorbed
However, for dual-stage amplifiers, keeping a constant ratio as above for each stage does not give a constant gain, as illustrated in FIG. 2 (prior art), showing the resulting WDM gain for a given launched power. The gain was measured using the saturating tone method, as described in D. M. Baney et al. xe2x80x9cWDM EDFA gain characterization with a reduced set of saturating channelsxe2x80x9d, IEEE Photon. Tech. Lett. vol 8, pp 1615-1617 (1996). In this method, the saturation level when several WDM channels are present at the input of the amplifier is replicated using a single saturating tone. The WDM gain is then measured over the operating bandwidth of the amplifier using a tunable laser. A saturating tone at a wavelength of 1550 nm was used for the measurements reported in FIG. 2, and the WDM gain was measured at 1540 nm. The ratio k for each stage was first set to the nominal operating pump powers and the power level of the saturating tone was adjusted to get the desired WDM gain of 20 dB at 1540 nm. The WDM gain was then measured at different power levels of the saturating tone while keeping the ratio for each stage constant by setting their respective launch pump power accordingly. As can be seen, the gain deviation reaches a value as high as 8 dB at low power values of the saturating tone, which is unacceptable for standard WDM applications.
It is therefore an object of the present invention to provide a gain-locked dual or multi-stage optical amplifier.
It is also an object of the invention to provide a method for amplifying an optical input signal while keeping the amplifying gain constant.
Accordingly, the present invention provides a gain-locked dual stage optical amplifier for amplifying an input signal. The amplifier includes an optical waveguide, wherein the input signal propagates. The optical waveguide has a first and a second amplifying stage, for consecutively amplifying the input signal.
A first pump source, coupled to the first amplifying stage for launching a first pump signal therein, is also provided. The first pump signal has an initial pump power Pi1. A fraction of the initial pump power Pi1 propagates through the first amplifying stage without being absorbed thereby, defining an unabsorbed pump signal having a pump power Pu. The amplifier also includes a second pump source coupled to the second amplifying stage for launching a second pump signal therein. The second pump signal has an initial pump power Pi2. 
Monitoring means are also provided for monitoring the initial pump power Pi1 of the first pump signal, the initial pump power Pi2 of the second pump signal and the pump power Pu of the unabsorbed pump signal, and controlling means are included for controlling the initial pump power Pi1 and Pi2 of the first and second pump signals. The controlling means set both of said initial pump power Pi1 and Pi2 to a same controlled value, depending on the pump power Pu of the unabsorbed pump signal.
Similarly, the present invention also provides a gain-locked multi-stage optical amplifier for amplifying an input signal. The amplifier has an optical waveguide, the input signal propagating therein. The optical waveguide has a first amplifying stage and a plurality of additional amplifying stages for consecutively amplifying the input signal.
The amplifier further has a first pump source coupled to the first amplifying stage for launching a first pump signal therein. The first pump signal has an initial pump power Pi1, a fraction of said initial pump power Pi1, propagating through the first amplifying stage without being absorbed thereby to define an unabsorbed pump signal having a pump power Pu. 
An additional pump source is provided for each of the additional amplifying stages. Each additional pump source is coupled to the corresponding amplifying stage for launching a secondary pump signal therein. Each secondary pump signal has an initial pump power Pin. 
Monitoring means for monitoring the initial pump power Pi1 of the first pump signal, the initial pump power Pin of each of the additional pump signals and the pump power Pu of the unabsorbed pump signal are included, as are controlling means for controlling the initial pump power Pi1 and Pin of the first pump signal and of each of the secondary pump signals. The controlling means sets the initial pump power Pi1 and all of the initial pump powers Pin to a same controlled value depending on the pump power Pu of the unabsorbed pump signal.
The present invention also provides a method for amplifying an optical input signal. The method includes the following steps:
a) propagating the input signal in an optical waveguide having a first and a second amplifying stage therein for consecutively amplifying said input signal;
b) launching a first pump signal in the first amplifying stage, the first pump signal having an initial pump power Pi, a fraction of said initial pump power Pi1 propagating through the first amplifying stage without being absorbed thereby to define an unabsorbed pump signal having a pump power Pu;
c) launching a second pump signal in the second amplifying stage, the second pump signal having an initial pump power Pi2;
d) monitoring the initial pump power Pi1 of the first pump signal, the initial pump power Pi2 of the second pump signal and the pump power Pu of the unabsorbed pump signal; and
e) controlling the initial pump power Pi1 and Pi2 of the first and second pump signals by setting both of said initial pump power Pi1 and Pi2 to a same controlled value depending on the pump power Pu of the unabsorbed pump signal.
The present invention and its advantages will be better understood upon reading the following non restrictive description of preferred embodiments thereof, made with reference to the accompanying drawings.